1. Field of the Invention
The present invention relates to a pneumatic tire in which a occurrence of voids in bead portions can be controlled and bead durability can be improved.
2. Description of the Related Art
In a pneumatic tire, as shown in FIG. 6(A), a carcass (a) is turned up around bead cores (b) to be secured thereto, and bead apex rubber (c) is filled between a ply main portion (a1) and each ply turnup portion (a2) of the carcass (a), thereby ensuring necessary bead rigidity.
In conventional heavy duty tires for trucks, buses and the like, the volume of the bead apex rubber (c) is increased to increase the bead rigidity and thereby to improve bead durability.
On the other hand, in recent years, a bead structure is revamped for weight reduction of the tire. FIG. 6(B) shows such a technique for reducing weight while improving the bead durability. In this technique, contrary to the above, the volume and height of bead apex rubber (c) are greatly decreased, and the height of a ply turnup portion (a2) is increased so that the ply turnup portion (a2) adjoins the ply main portion (a1). (hereinafter, referred to as a new bead structure).
In this new bead structure, the bead apex rubber (c) requires higher strength and higher rigidity than in the conventional structure. Therefore, the carbon black and/or crosslinking agent are increased. However, when the amount of carbon black is increased, rubber flowing during vulcanizing the tire becomes insufficient because of high viscosity, and, in the vulcanized tire, voids are liable to occur near the radially outer end (e) of the bead apex rubber (c). When the large amount of crosslinking agent is added, quality of a product tends to be impaired because rubber scorching may occur during extruding the rubber.
In the new bead structure, further, the height (h) is set as small as possible for the bead durability. However, as a degree of bending of the carcass ply turnup portion (a2) occurring below the adjacent region (g) is greatly increased due to the decreased bead apex rubber volume, its spring-back force during vulcanization is strong, and thus a tendency of occurrence of voids is further increased as the height (h) is decreased.